Many light years ago I have the actual listings for each pre cursor, so here its is from rough pieces of info:

Its clear that after one or two letter prefixes, the same letter prefix's were used by Maltby, Fazakerley & BSA Shirley.
A to Z then AA, AB to AZ, then BA to BZ, CA to CZ.
Maltby rifle serial numbers commence with a number '1', Fazakerley with a '2' and Shirley with a '3', e.g. 1xxxx for Maltby, 2xxxx for Fazakerley and for Shirley, 3xxxx, after the letter prefix.

Suffice it to say, they definately meant something to the QM masses out there in those days who at a glance could say what was in or what was going to go in a chest after its manufacture at one of the houese of excellence.